Add *mono variants to pip package

[maxfrei750]

When using mitsuba directly, self-compilation is a convenient way to add additional variants. Even deployment as part of bigger projects can be made convenient for end users, when using docker, since the compilation can be done during build time.

Unfortunately, docker does not yet allow the use of Optix on Windows yet (see https://forums.developer.nvidia.com/t/optix-on-wsl-windows-subsystem-for-linux/140297/15), which breaks cuda variants of mitsuba.

Therefore, the easiest way to deploy mitsuba on Windows is currently via pip. However, this limits the available variants. While I understand that it is not feasible to include all possible variants, I'd like to propose to add the two *mono variants

        "cuda_ad_mono",
        "llvm_ad_mono",

since I assume that they are the most popular, apart from the RGB variants.